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New Scientist Live

Neutrinos caught ‘shape shifting’ in new way

By David Shiga

Neutrinos have been caught spontaneously flip-flopping from one type to another in a way never previously seen. Further observations of this behaviour may shed light on how matter came to dominate over antimatter in the universe.

Neutrinos are among the most slippery particles known to physics. They rarely interact with ordinary matter, but massive experiments have been set up to detect the flashes of light produced when they do.

There are three known types, or flavours, of neutrino&colon; electron, muon, and tau. Several experiments have found evidence that some flavours can spontaneously change into others, a phenomenon called neutrino oscillations. For example muon neutrinos can change into tau neutrinos.

Now, results from a Japanese experiment called T2K have tentatively added a new kind of transformation to the list of allowed types – the metamorphosis of muon neutrinos into electron neutrinos.

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T2K generates muon neutrinos at the J-PARC accelerator in Tokai, Japan, and sends them in a beam towards the Super-Kamiokande neutrino detector in Kamioka, 295 kilometres away. It began operating in February 2010 and stopped gathering data in March, when Japan was rocked by the magnitude-9 megaquake.

Still tentative

On Wednesday, the team announced that six of the muon neutrinos that started off at J-PARC appear to have transformed into electron neutrinos before reaching Super-Kamiokande, where they were detected. This is the first time anyone has seen electron neutrinos show up in a beam of particles that started off as muon neutrinos.

“It shows the power of our experimental design that with only 2 per cent of our design data we are already the most sensitive experiment in the world for looking for this new type of oscillation,” says T2K spokesperson Takashi Kobayashi of Japan’s KEK particle physics laboratory.

However, the result is still tentative because of the small number of events seen and because of the possibility – considered rare – that muon neutrinos could be misidentified as electron neutrinos. Still, the researchers say experimental errors should give only 1.5 false events in the amount of data they analysed. There is only a 0.7 per cent chance of producing six false events.

Antimatter counterparts

The transformations appear to be happening relatively frequently. That means researchers will be able to quickly accumulate more events – once the experiment begins running again. The earthquake threw the accelerator used to make the neutrinos out of alignment. After adjustments are made, researchers hope to restart the experiment by year’s end.

The researchers may eventually rerun the experiment with a beam of muon antineutrinos to see if their behaviour differs from their normal-matter counterparts.

If differences are found, it could help explain why there is a preponderance of matter in the universe. Standard theories say that matter and antimatter were created in equal amounts in the universe’s first instants, but for unknown reasons, matter prevailed.

Skew the balance

Reactions involving neutrinos and antineutrinos in the early universe could have skewed the ratio of matter and antimatter production, leading to our matter-dominated universe. “You need some new laws of physics that aren’t the same for matter and antimatter, and neutrino physics is one place you could put such laws,” says David Wark of Imperial College London, who is a member of the T2K collaboration.

The US-based MiniBoone experiment recently found hints of an antimatter version of the oscillation seen by T2K. MiniBoone found signs that muon antineutrinos sometimes change into electron antineutrinos.

But physicists are still puzzling over the MiniBoone results. Based on the experiment’s design, it should not have seen oscillations unless there are one or more extra types of neutrino that are sterile, meaning they are even more averse to interacting with matter than regular neutrinos.

By contrast, the T2K result can be accommodated without invoking sterile neutrinos.

The T2K collaboration has submitted the new results (pdf) to Physical Review Letters.